White-nose syndrome has invaded Fern Cave National Wildlife Refuge, home to more than 1 million endangered gray bats and other vulnerable species.

The world's largest wintering colony of gray bats may be under attack from white-nose syndrome, federal wildlife authorities reported Monday, potentially spelling doom for the already-endangered species. More than 1 million gray bats live in Alabama's Fern Cave National Wildlife Refuge, along with large numbers of bats from other species, and white-nose syndrome is notorious for a mortality rate that can reach 100 percent.

The good news is that white-nose syndrome (WNS) has not been proven to kill gray bats, which occupy a limited range in limestone caves of the U.S. Southeast. The bad news is that it kills many similar American bat species en masse, and scientists know gray bats are at least susceptible to WNS infections. Plus, the disease is new enough — having emerged just seven years ago — that a lack of proof doesn't necessarily mean anything.

"With over a million hibernating gray bats, Fern Cave is undoubtedly the single most significant hibernaculum for the species," says Paul McKenzie of the U.S. Fish and Wildlife Service. "Although mass mortality of gray bats has not yet been confirmed from any WNS-infected caves in which the species hibernates, the documentation of the disease from Fern Cave is extremely alarming and could be catastrophic."

Gray bats form massive, year-round colonies in a small number of caves, making their overall population highly vulnerable. They were federally listed as an endangered species in 1976, and while they have made gains since then, their slow reproduction rate makes it hard to rebound quickly. No gray bats at Fern Cave have been found with the telltale white fuzz of WNS, but the disease was confirmed in several tri-colored bats there. The cave is also home to roughly 1 million Indiana bats, another endangered species.

WNS has spread to 22 U.S. states and five Canadian provinces since its 2006 debut in New York, and shows no signs of slowing down. 2012 marked its arrival in Alabama, Iowa and Missouri — including the first confirmed case west of the Mississippi River — and this spring it pushed into Illinois, South Carolina and Georgia. It has already killed about 6 million North American bats, according to the FWS, threatening economic as well as ecological upheaval across the continent. According to a 2011 study, bats save the U.S. agricultural industry more than $3.7 billion per year just by eating crop pests.

The spread of white-nose syndrome since 2006. Click map to enlarge. (Image: FWS)

Caused by the previously unknown fungus Geomyces destructans, WNS doesn't seem to kill bats directly. Instead, it wakes them up from hibernation too early, leaving them to fruitlessly search for insects to eat during winter. Bats killed by WNS often have empty stomachs and depleted fat reserves, suggesting they starved to death.

Although scientists aren't sure of G. destructans' origins, they suspect it's an invasive species from Europe, where native bats seem immune to the effects of similar cave fungi. They may have evolved a defense mechanism, but when G. destructans arrived in America — possibly by hitchhiking on the clothes or equipment of trans-Atlantic spelunkers — it found a continent full of helpless hibernating hosts. The fungus is transmitted mainly from bat to bat, but scientists think humans may still be aiding its spread by inadvertently carrying spores on their shoes, clothes and caving equipment.

Fern Cave National Wildlife Refuge includes 199 acres of forested hills underlain by a vast cave with five hidden entrances, according to the FWS. Four of those entrances are in the refuge, and one is owned and managed by the Southeastern Cave Conservancy Inc. (SCCi), which helps the FWS monitor local bat populations. It's unclear if humans played a role in bringing G. destructans to the cave, but the FWS notes that access is "extremely difficult," and expert cavers describe its 15-mile tunnels and 450-foot dropoffs as "a vertical and horizontal maze." The WNS-infected bats were found during recent surveillance trips by the FWS, the SCCi and the National Speleological Society (NSS).

"Since discovering Fern Cave in 1961, members of the National Speleological Society have worked hard to protect it," says Steve Pitts, SCCi Fern Cave property manager, in a press release. "For over 30 years, NSS members have also been key U.S. Fish and Wildlife Service partners in the effort to protect Fern's gray bat colony. It's a huge blow to all of us who love Fern Cave to know that WNS is now there. We hope the gray bats will survive."